This invention is concerned with materials that are used to develop electrostatic images on a photoconductor. With the increased use of plain paper copiers, development powders have enjoyed an increased popularity over liquid toners. Along with the increased use of development powders, magnetic brush units are becoming increasingly popular as opposed to cascading methods. Development powders used with magnetic brush units usually have an iron powder, which serves as the carrier material, and a toner that adheres to the carrier particle prior to contact with an electrostatic image. The toner usually consists of a resinous member and a coloring agent. Inexpensive, untreated iron powders cannot be used in magnetic brush systems since such iron does not have sufficient stability toward rusting and has color and triboelectric charging properties adversely effected by variable humidity conditions.
Untreated carrier particles which are employed in automatic copy machines also have carrier filming problems due to the mechanical rubbing of the carrier surface with the soft toner resins. The gradual accumulation of permanently attached film impairs the normal triboelectric charging of the toner particles in the toner mix. As a result, the toner is either less highly charged or sometimes oppositely charged giving rise to poor copy quality with a high degree of background.
In order to solve these problems, those in the art have resorted to a variety of methods for altering the carrier particle such as chemical plating and coating of the iron particles with polymers, oils, waxes and the like. Recently, there has been disclosed methods of treating the carrier particles with acids, such as fluorinated carboxylic acids, in order to prevent oxidation and modify the charge to mass ratio.
In the literature, several types of plastic coatings and electroplatings of the carrier have been suggested to overcome the filming problems. Most of the prior art coating methods result in high cost and/or have other disadvantages such as yielding improper triboelectric charge properties and imparting a very high electrical resistance to the carrier that reduces its development electrode effect and results in poorly filled-in large image areas.
Of late, certain methods have been disclosed wherein increase of the charge to mass ratio of the development powder has been achieved. This is particularly true of the treatment of the carrier particle with perfluoro carboxylic acids; however, the charge to mass ratio is still not sufficient large for a relatively rapid loss of charge to mass occurs upon use of such treated carrier particles.